1. Field of the Invention
This invention relates to a method for preventing seismic liquefaction of loose alluvial low ground or loose reclaimed ground (hereafter two of them combined called “loose fine grained layer”) in urbanized areas vulnerable to seismic liquefaction, by lowering the saturation degree of groundwater in said loose fine grained layer after pumping groundwater out of said loose fine grained layer.
The invention is applicable to very wide and extensive field of use. Among those fields of use to which this invention is particularly applicable is the prevention of destructive seismic liquefaction of said fine grained layer in a built-up urbanized area including a harbor area vulnerable to seismic liquefaction which combined with intense tremor causes such damage as collapse of buildings, bridges, viaducts, piers, wharves, and/or outbreaks of fire due to the tremor and catastrophic spreading on of the fire due to a complete lack of water for fire fighting troops as a result of supply water pipes being torn into pieces caused by the pulling, pushing or twisting motion of the ground due to seismic liquefaction.
2. Description of the Prior Art
Liquefaction of ground is a peculiar phenomenon that occurs when a loose fine grained layer saturated with groundwater is shaken strongly by an earthquake.
This type of phenomenon can be observed when the volume of dry sand loosely filled in a container decreases when the container is shaken strongly because the pore volume of the sand decreases by the shaking down motion.
A similar phenomenon takes place when a dry loose sandy stratum is shaken strongly by an earthquake, the ground settles down because the pore volume of the ground decreases.
In the case when a dry loose sandy ground is shaken strongly, any severe damage may not be caused by it, even though appreciable settlement of the ground surface may take place.
However, in the case when said loose fine grained layer where the volume of porous void in said stratum is filled with groundwater which is called “pore water” or when said layer is saturated with the pore water which is not as compressible as air is, the motion to decrease pore volume due to strong shaking action causes a sudden rise of pore water pressure much in excess of the normal hydrostatic pressure causing the effective contact pressure between soil grains which is called “effective overburden pressure” enacted by the weight of the ground less the buoyancy of a submerged portion of ground above a depth level, to diminish to null so as to create a state as though soil grains drift in the pore water.
This peculiar phenomenon is called liquefaction of ground.
When seismic liquefaction of ground occurs, any obstacle in the ground lighter in unit weight than the ground floats up and anything heavier in unit weight than the ground sinks down.
The liquefied ground loses its bearing capacity to cause such a destructive damage as collapse of buildings, bridges, viaducts, wharfs, piers or other types of structure.
Also such a disastrous hazard of an overwhelming fire, a great many casualties and a tremendous loss of properties may be caused by break open buried lifelines of pipes and ducts for feeding water, gas, electric power or for communication lines as the liquefied ground flows slowly toward a low side even on a slope of very slight gradient and the solid ground above the groundwater table where the pipes or ducts of lifelines are buried in it moves together with the flowing liquefied ground beneath the ground water table which induces compression or extraction of the solid ground to tear or crush forcibly the buried lifelines.
The likewise break open of a sub-aqueous tunnel leading into the low level areas may cause deadly flood in the low level area caused by high tide or Tsunami induced by the movement of active faults below sea floor to torture many helpless people by drowning or starving.
The physical property of the ground vulnerable to seismic liquefaction was defined to be that (1) relative density 75% or less, (2) grain-size uniformity factor 10 or less (3) 50% grain diameter D50 0.074 mm to 2.0 mm and that (4) effective overburden pressure 0.20 MPa (2 kgf/sq.cm) or less.
However, in violent Hyogoken Nambu Great Earthquake of 1995, liquefaction occurred in the ground of sandy gravel where D50 is much larger than 2.0 mm and in the ground in a loose fill with an “apparent cohesion” containing an appreciable amount of fine particles smaller than 0.074 mm in diameter which behaves like a cohesive soil while it is not fully saturated with the pore water contained in it, its apparent cohesion is lost when it is fully saturated with the pore water in it.
Such a ground with an apparent cohesion is vulnerable to seismic liquefaction, it is found in many cases where the reclaimed fill overlaying the soft cohesive layer called New Bay Mud is prevailing along the sea shores and below the sea bed of San Francisco Bay and California Bay or in the loose fill made of disintegrated soil dredged out of New Bay Mud.
Severe liquefaction of ground occurred in the above mentioned loose fill caused by recent intense earthquakes including Loma Prieta Earthquake of 1989.
The prior countermeasures for preventing seismic liquefaction of ground are, (1) methods to improve the ground so that liquefaction of it does not occur even though it is shaken by an intense earthquake and (2) methods to renew or to retrofit the existing structures or underground utilities so that they are not fatally damaged even when a liquefaction of ground occurs.
Among the aforementioned countermeasures by improving the property of ground is A. a method to increase the density of ground by compacting the ground, B. a method to solidify ground by injecting chemical fluids into the ground, C. a method to replace the ground with better soil and D. a method to lower the saturation degree of pore water contained in the ground.
The prior method to increase the density of ground by compacting the ground by means of powerful vibro-hammers or impact hammers mounted on a large crawler-mount pile driving rig and the like is not only very expensive but also extremely difficult to apply in a built-up urban area or in a harbor area where there is not any vacant space which is not occupied by containers such an activity as busy road traffic nor occupied by containers and/or container lifting cranes installed on wharfs and piers because it requires detouring of traffic or removal of container and cranes to make room for the large pile driving rig with its outriggers fully extended to be ready for its work.
The method using the above mentioned tall large rigs is neither applicable to the place with narrow space nor to the place below an overpass girder where the head clearance is low.
The application of the said method to increase the density of ground to built-up urban areas or to harbor areas is impracticable.
The prior method of solidifying the ground or of replacing the ground with better soil is more expensive than said method of increasing density of ground because the former requires a large amount of chemicals of high price and the latter a large amount of good soil of high price and the cost for removing the original ground and additional cost of a borrow pit and carrying good soil from the borrow pit to refilling site.
The foregoing methods for preventing seismic liquefaction of ground by increasing the density of ground, by solidifying the ground, by replacing the ground with good soil are much too expensive and their application covering wide areas is impracticable because it requires a huge amount of funds which is much in excess of the fund rising ability of any organization concerned.
Prior methods proposed to renew and/or to retrofit the existing structures or underground utilities requires a tremendous amount of funding because there are a great many quantity of the existing structures and/or underground utilities to be renewed and/or retrofitted in a built-up urban area.
Therefore, the application of these methods is practicable only in a very limited scope.
The three billion US dollar long range seismic retrofit program presently being enforced by California Department of Transportation (Caltran) to reinforce eighteen toll bridges spanning across San Francisco Bay and California Bay is an example of said method for preventing seismic damage caused principally by liquefaction of ground where the funds required for said retrofit program is being raised by issuing long-term bonds to be refunded by allocating an important portion of the reserve raised out of toll revenue.
The above Caltran's retrofit program is an example where the object of application is limited solely to the toll bridges financed by their toll revenue.
Whereas there are a great many aging structures and/or deteriorated buried life lines of pipes and ducts required to be renewed and/or retrofitted in the urbanized areas in the State of California alone.
The above description regarding the program for preventing seismic damage provided in the American Continents including said three billion dollar seismic retrofit program has been quoted from the literatures written by and the information afforded by Ben C. Gerwick, Jr., Honorary Member of American Society of Civil Engineers who has been assigned by Caltran to be a consulting engineer playing an important role in engineering the Caltran's retrofit program.
Prior methods proposed for lowering the saturation degree of pore water (water in porous voids of ground) contained in a ground where the saturation degree is defined to be the ratio in percent of the volume of pore water to the total volume of porous void in the ground is further divided into the method of lowering ground water level by means of deep wells and the like, and the method of blowing compressed air into the ground.
By said method utilizing deep wells or the like, the ground water is pumped out for lowering the groundwater table.
This method involves the problem of land subsidence due to the consolidation of soft strata caused by the lowering of the ground water table and thus its application to built-up urban areas is impracticable.
The present invention belongs to said method D among the countermeasures for preventing seismic liquefaction of ground without any of the disadvantages associated with the prior methods of lowering the saturation degree of pore water in the ground.
Those methods patented by the United States Patent and Trademark Office that fall into the above mentioned category and sorted out of the U.S. patent data base by the courtesy of Jotaro Iwabuchi, Ph.D. PE meeting the demand of the claimant for the patent of the present invention, are as follows: U.S. Pat. No. 5,927,907 “Method and apparatus for preventing liquefaction of ground caused by earthquake”, U.S. Pat. No. 5,868,525 “Method of preventing damage to loose sand ground or sandy ground due to seismic liquefaction phenomenon, and of restoration of disaster-stricken ground”, U.S. Pat. No. 5,800,090 “Apparatus and method for liquefaction remedy of liquefiable soils”, and U.S. Pat. No. 5,779,397 “Method of improving against vibration and liquefaction”.
Those Japanese patented methods which fall in said category were sorted also by the courtesy of Jotaro Iwabuchi, Ph.D. PE out of the data base of Electronic Library of Japanese Patent Board are: Published JP-A-2001-123438 “Method for preventing seismic liquefaction of ground in urbanized area by injecting air-solved water or compressed air and facilities used in the method”, JP-A-2000-345549 “Method for preventing liquefaction of ground by making air-solved water permeate into ground”, Published JP-A-2001-1930498 “Method for improving ground and quality of water by injecting gas solved in water”, Published JP-A-H10-102473 “Method for preventing liquefaction of sand and sandy ground” and Published JP-A-H06-57730 “Method for preventing liquefaction of ground by using burnt ash”.
Among the above mentioned patents, the U.S. Pat. No. 5,927,907 and the Japanese Patent of Published JP-A-H10-338989, Published JP-A-2001-123438 and Published JP-A-2000-345549 are invented by and granted to the claimant for a patent of the present invention.
However, every one of the above quoted patented methods for preventing seismic liquefaction of ground by lowering the saturation degree in ground has the drawback as described in the following paragraph.
The main feature common in the above quoted patented methods is to form an air mixed zone in the ground by such a means of injecting compressed air or by a similar means.
However, every one of the above quoted patented methods has a disadvantage in that the air-mixed zone thus formed develops in a limited extent because the countless tiny air bubbles swarmed in said air mixed zone concentrate around the outlet of the source of feeding said air bubbles to minimize further expansion of the air mixed zone.
According to the data base CLIPPEDIMAGE-JA404131427A, Patent JP-A-H04-131427: “Prevention of ground from liquefaction”, PUBN DATE: May 6, 1992, Inventors: K. Tomaoki, N. Mori, M. Sato and Y. Yoshimi, IPC: E02D27/34 US-CL-CURRENT: 40P5/267, ABSTRACT: To prevent liquefaction of the ground with the reduced underground water level in the upper water-bearing stratum by providing a cut-off wall-impermeable layer and making a wall reaching the lower water-bearing stratum in the inside of the cut-off wall conducting the underground water of the upper water-bearing stratum to the lower water-bearing stratum through the well (to aerate the upper water-bearing stratum so as to lower the saturation degree of the upper water-bearing stratum for making it being not liquefied at the time of a violent earthquake).
The above mentioned method for preventing liquefaction of ground, where the description in parentheses was added for making the effect achieved by the method clear, is applicable to a limited extent of the ground within the width between the cut-off walls beneath the building before it is built.
There are a number of patented methods for preventing seismic liquefaction of ground similar to the one described above.
However, they are applicable to a limited extent of ground within limited spaces.
There are many prototype examples where the structure built on pneumatic caissons surrounded by loose fine grained strata are vulnerable to seismic liquefaction of said loose ground.
The earliest recorded typical example is the Bandai Bridge based on pneumatic caissons built in 1947 supporting the main spans of continuous arch endured Niigata Earthquake of 1964 when it was shaken by the tremor of 0.3 g (g is the acceleration of gravity) in maximum horizontal acceleration without any damage affecting the loading capacity of its main arch spans while many other structures fatally damaged due to the seismic liquefaction of the loose sandy layer several meter in depth below groundwater table.
According to the theory of groundwater hydrology such tiny air bubbles smaller in diameter than 1 mm closed in the pore voids of a loose sandy layer stay in there permanently as long as no such a radical change in groundwater as drying up by heating or as a turbulent flow where the rate of flow much in excess of the maximum rate of steady flow were to occur.
A typical example to verify what is described above was achieved by the soil tests made on undisturbed samples taken out of the loose fine grained layer below groundwater table, which was vulnerable to seismic liquefaction if it were saturated with water, at the position 1.5 m apart from the outside surface of one of circular pneumatic caissons supporting the piers higher than 20 m above the ground surface of Kashima Viaduct on Sanyo Shinkansen Rail Line in Osaka City.
A series of laboratory tests made on said samples of loose sandy soil were made carefully to determine the value of saturation degree of them.
Saturation degree is a volumetric ratio in % of pore water to the total pore voids.
The values of saturation degree thus examined were in the range from 83.5% to 92.4% and it verified said theory.
The present invention is composed for the object to solve those problems that have not been solved by the prior method for preventing seismic liquefaction of ground.
Therefore, the present invention will be composed for solving those problems in the manner as summarized below.